S0NofaGUN wrote:64 bit
The directory for oblivion is C:\Program Files (x86)\Oblivion
I have two guesses here:
1. Installers are REALLY really bad at handling the () part of the file path. MS implemented it in a real bad hodge podgy way. and really, it was absolutely unnecessary to begin with, there is no reason you cannot install both 32bit and 64bit programs into C:\program files\ So maybe having it in that directory it messing up the installer.
2. windows vista and win7 have introduced very severe limitations on what programs can do to files in program files directory as a "security measure"... this is why google chrome simply says "to heck with that" and installs into your AppData directory instead

.
There is a good chance that just being in there is messing it up. Windows thinks "oh my god, nehrim install is modifying program files, it must be a virus, lets block it". and it does.
Disabling UAC can also help. My brother on win7x64 could not save files from a browser (IE, firefox, or chrome) into his program files directory (he wanted to download an installer there, but afterwards we just tried to save a jpg file). He had to disable UAC before being allowed to save a file there from within a browser.
My recommendation is that you uninstall oblivion, then install it into C:\Games\Oblivion
then install Nehrim into C:\Games\Nehrim
In fact, NEVER install a game into program files, all games should be installed into a custom directory such as <drive letter>:\Games\<game name>